Fix pattern for needle tag

[perlpunk]

Developer console showed me this error:

Pattern attribute value ^[A-Za-z0-9-_]+$ is not a valid regular expression:
Uncaught SyntaxError: Invalid regular expression: /^[A-Za-z0-9-_]+$/v:
Invalid character class

Apparently dashes have to be escaped. Moving it to the end of the character class (this would have fixed it for a perl regex) did not help.

[perlpunk]

I guess a test should be possible. The "Add" button should not be clickable if the pattern doesn't match.

[Martchus]

I guess it would make sense to change the title as well but the change itself looks good. Testing could be done in the Selenium-based test of the needle editor but is likely not required for this trivial change.

[perlpunk]

Since we actually never tested this, I added a test for it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.98%. Comparing base (9a22ad5) to head (18f2730).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6024   +/-   ##
=======================================
  Coverage   98.98%   98.98%           
=======================================
  Files         395      395           
  Lines       39344    39354   +10     
=======================================
+ Hits        38944    38954   +10     
  Misses        400      400           

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